Various types of intrusion detectors are presently used for non-contact intrusion detection. These include passive detectors, particularly passive infra-red detectors, such as those described and claimed in the following U.S. Pat. Nos. 4,258,255; 4,447,726; 4,484,075; 4,604,524; 4,709,153; 4,725,768; 4,242,669; 4,982,094; 5,084,696; 5,077,549.
Active detectors, which generate a radiation beam and sense the presence or absence of reflection thereof are also employed for this purpose.
Passive infra-red detectors, whose use is extremely widespread for indoor applications have not been successfully employed in outdoor applications. A principle reason is that the region relatively close to the detector is extremely sensitive to spurious inputs, such as temperature changes, wind, rain, hail, birds and other creatures, producing an unacceptable false alarm rate.
Both active and passive non-contact detectors have the disadvantage for outdoor applications that they are normally readily noticeable and thus are subject to vandalism, rendering them wholly or partially inoperative.